It is conventional that a plurality of function modules may be provided in a motor vehicle, where they are allocated to engine management or vehicle body electronics, for example, and special vehicle equipment such as a radio, car phone or navigation system. It is also conventional that the individual function modules may be connected to a bus system, e.g., a CAN bus, for data exchange and for triggering, and a separate bus may be provided for each of the groups indicated above. The bus systems may be interconnected via a gateway for exchanging data and control commands. The function modules that require a high-speed data exchange, e.g., the control units for the engine and transmission, as well as other components that are required for functionality and safety, are preferably connected to a so-called high-speed bus, and the less time-critical components, e.g., those that support special equipment, are connected to a so-called low-speed bus or comfort bus.
The function modules and/or control units include a controller and a transceiver, the main task of the transceiver being to create a physical interface with the bus system. The level of the messages received and/or to be sent is adjusted in the transceiver.
Since transmission rates in the bus systems are relatively high, reflection at the transition points must be reduced. In principle two different approaches are conventional.
In decentralized termination, a terminating resistor whose resistance is equal to the characteristic impedance of the bus line is provided on both ends of the bus system, the two characteristic impedances being provided in the last control units at the end of the bus system. A typical value for the characteristic impedance is 120Ω for a twisted double line, for example. One problem with decentralized termination is that the design is not always known in advance. It is therefore also not known where the end of the bus system is located. In addition, with a star-shaped bus structure, there is no clearly discernible beginning and end.
Therefore, centralized termination is an alternative to decentralized termination. In centralized termination, the termination occurs centrally, e.g., in a control unit, typically with half the characteristic impedance being chosen as the terminating resistance. This is a reasonable compromise to minimize reflections centrally.
The two concepts for termination have in common that the control units not involved in termination of the bus system are terminated using the highest possible resistance, e.g., in the kiloohm range.
The particular terminating resistors are provided together with elements of a supporting network and/or filters that may be necessary as discrete components on the transceiver. Owing to the fact that a central control unit may be a component of a centralized or decentralized termination or may not contribute to termination at all, the terminating resistor must be adapted, depending on the bus structure. This may be accomplished manually by replacing the resistors, or the same control unit may be manufactured in three variants having different terminating resistors, which is very complex and expensive.
German Published Patent Application No. 198 54 117 describes a circuit arrangement for communication with a plurality of electric components in a motor vehicle, including at least one bus for transmission of data between the components connected via network nodes and at least one terminating resistor, the at least one terminating resistor having an adjustable resistance value. The resistance of the terminating resistor may be adjusted electronically and is designed as a digital potentiometer, for example. In addition, it is proposed that all network nodes be allocated an adjustable terminating resistor. Depending on the type of adjustable resistors used, the adaptation of the total resistance may be accomplished in various manners. In an exemplary embodiment, one of the network nodes analyzes the status messages of the other network nodes connected to the bus and automatically adjusts the resistance of its terminating resistor. The adjustment may also be accomplished automatically by an internal control unit on the bus which measures the total resistance and then adjusts the terminating resistances of all bus devices. Finally, the adjustment may also be performed by an external diagnostic device connected to the bus, either at the time of manufacture, e.g., after assembly of all function modules on a bus, or while being serviced in the shop for maintenance or for readjustment after installation or removal of modules. This allows simplifications in manufacturing and in subsequent modifications of motor vehicles, so that only a small number of control unit variants is required. When the adjustable resistors are integrated into the network nodes, separate assembly with terminating resistors is superfluous.
German Published Patent Application No. 100 20 142 describes a bus system including a bus structure to which electric components are connected, an error counter and a terminating resistor being assigned to the components, the terminating resistor being designed to be adjustable and variable as a function of the error density detected by the error counter. The adjustable terminating resistor may be designed as a digital potentiometer and is situated in the electronic component.